In hindsight it's becoming clearer and clearer to me that candidates like Obama, those who make outrageous promises they have no intention of fulfilling (I know all politicians do this to some degree but Obama took it to a whole new level and many on both sides have been following in his footsteps since), are destined to destroy their party for a while especially if they serve two terms. They enrage the other side and they disappoint their own voters. Had Trump won reelection I suspect he would have left the Republican Party crippled for a few years in much the same way that the Democratic Party seems in Obama's aftermath.
I don't know what the country or the Democratic Party would look like in an alternate timeline, and it's entirely possible that the same consultant class of privileged liberal progressives would've taken over the party after her reelection and put the Democrats in much the same situation that they are in now. That said, I find it hard to believe that if 2008 Hillary had won there would have been quite the same immediate backlash there was to Obama.
In a way, Bill Clinton had the same problem. He didn't run on an "ambitiously progressive" agenda, but his 1992 campaign was objectively one of the most effective ones in history. Then his approval took a dive shortly after he took office (Whitewater) and then of course 1994 happened. And then Lewinsky.
And while people didn't approve of Republican overreach in impeaching Clinton, neither did they feel like they could trust Clinton as someone with integrity. So long Louisiana, Arkansas, Missouri, Tennessee, Kentucky, West Virginia... and later Iowa and Ohio.
But still, that "permanent Democratic majority" will come any day now!